It isn't a sequel, but a spin-off.. I watched Torchwood before I watched Doctor Who, at which point I watched the reboot before I watched the classic series.
As for books.. the one that really comes to mind is that I read The Empty Chair from Jeffrey Deaver's "Lincoln Rhyme" crime novel series first, then went back and read the previous ones.
And, television, again not sequels per se. I have a hard and fast rule about not watching any show that's less than two seasons old because I've been burned too many times by falling in love with a show and it getting canceled with no real ending. Which can happen with older shows, too, I know. I'm looking at you, FOX. Lie to Me*? Really? So, I pay no attention to what's coming out at any given time. Then, a couple of years down the line, someone'll be all "Oh, you've gotta watch this! You'll love it." And sometimes I'll watch it and love it and I go back and catch up on what I've missed.